Not applicable.
Not applicable.
1. Field of the Invention
The invention relates to an objective with crystal lenses. Such objectives have been known for over a hundred years as Carl Zeiss apochromatic microscope objectives with fluorspar (CaF2) lenses.
In recent times, refractive projection objectives for microlithography in the DUV at wavelengths of 248 or 193 nm have been constructed, and contain lenses of quartz glass and CaF2.
2. Discussion of Relevant Art
An optical system is known from German Democratic Republic Patent DD 222 426 B5, with optical glasses and BaF2 single crystal as optical media, and can be used for wavelengths of 150 up to 104 nm. The embodiment example is a planapochromat for 480-800 nm with several different glasses and BaF2.
The choice of materials for UV microlithographic objectives (centered on 248 nm wavelength) is described in G. Roblin, J. Optics (Paris), 15 (1984), pp. 281-285.
The outcome was that only combinations of quartz glass with CaF2 or LiF are graded as usable.
In U. Behringer, F+M (Munich) 107 (1999), 57-60, fluorides such as CaF2, MgF2 and LiF are described as suitable for 157 nm microlithography, with reservations regarding the double refraction of MgF2 and regarding the handling of LiF.
In K. F. Walsh et al., SPIE Vol. 774 (1987), 155-159, excimer lasers for wavelengths of 248, 193 and 157 nm are presented, among other things. For 248 nm quartz glass, CaF2, BaF2, and MgF2 are named as the only applicable lens materials. For wavelengths below 248 nm, quartz glass is expected to be the only one material which is usable.
A catadioptric 1:1 projection objective for microlithography at 248 nm is described in U.S. Pat. No. 5,031,977, and contains a concave mirror, a quartz glass lens, an LiF lens, and two CaF2 deflecting prisms. Neither arguments for choice of materials are given nor information on modifications of the special construction.
However, the absorption edge of quartz glass is situated close to 157 nm. CaF2 still usefully transmits at 157 nm, but has too high a dispersion for a purely CaF2 objective for microlithography, even for a spectrally narrowed F2 excimer laser. Heretofore objectives for wavelengths below 193 nm are therefore known only as catadioptric (see German Patent Document DE 196 39 586 A of the same Inventor, and U.S. Prov. Apln. Ser. No. 60/094579 of Jul. 29, 1998) or catoptric (see U.S. Pat. No. 5,686,728) systems. Here U.S. Pat. No. 5,686,728 gives a purely mirror objective for VUV microlithography with a 126 nm, 146 nm, or 157 nm excimer laser, for example.
The invention has as its object an alternative concept of an objective with a new composition of material which opens up new possibilities of application, particularly in microlithography at low wavelengths.
This object is attained by an objective having a plurality of lenses made of at least two different crystals. This object is also attained by a microlithographic projection objective corrected for illumination with a F2 excimer laser at 157 nm, wherein the projection objective is purely refractive and comprises a plurality of lenses of a material selected from the group consisting of BaF2, SrF2, NaF, LiF, and KF. This object is also attained by a projection exposure equipment having a light source including an excimer laser with 100-160 nm wavelength, an illumination system including refractive optical elements comprising one or more fluorides, a reticle positioning and movement system, a projection objective with a plurality of lenses comprising at least two of crystalline materials selected from the group consisting of CaF2, BaF2, LiF, NaF, SrF2, KF and amorphous BeF2, and an object positioning and movement system. A process according to the invention for the production of microstructured components provides that a substrate provided with a photosensitive layer is exposed to ultraviolet light by means of a mask and a projection exposure equipment and if necessary after development, the photosensitive layer is structured corresponding to a pattern contained on the mask.
The invention starts from the discovery that novel kinds of objective properties can be provided by the use of two different crystals in an objective. In particular, included in this is the possibility of achromatizing at low wavelengths, at which each known glass, including quartz glass, absorbs strongly. The existing reservations in microlithography against BaF2 relate to 248 nm and quartz glass as the partner.
Alkali and alkaline earth halides, especially the fluorides, and also other fluorides, are known as optical materials. Their partially difficult properties as materials have however led to their outstanding transmission properties in the deep UV being made use of only in an adjunct manner. It has been shown according to the invention that optical microlithography can be extended down to about 100 nm wavelength with these and similar materials.
A pair of materials for the achromatization of 157 nm optics can be provided for the first time by the pairing of two fluoride crystals, particularly of CaF2, BaF2, SrF2, LiF, NaF, or KF, but also of mixed crystal fluorides. The materials are already known in optical manufacture, as evidenced by the cited state of the art. Barium fluoride, strontium fluoride, or sodium fluoride are preferably used for negative lenses, and indeed only for individual ones, as this can be sufficient. Calcium fluoride then finds application not only for the positive lenses, but also for the remaining negative lenses.
It is particularly advantageous that numerical apertures greater than 0.5 are attained, even at 157 nm. The following example with a numerical aperture of 0.8 reveals this clearly. The advance in resolution of EUV microlithography caused by about 1/10 of the wavelength is thereby partially compensated, since three times the NA is attained. As against 193 mm, the resolution can be nearly halved with 109 nm, since the level of the NA is retained. For the accuracy of processing, the invented technology has dramatic advantages over EUVL caused by tenfold the wavelength.
The stitching process (exposure of the chips by columns) according to an advantageous feature of the invention has recently been talked about for microlithography at very low wavelengths, and permits image fields of reduced size as rectangles with a moderate aspect ratio and thus makes possible a dramatic reduction in size of the objectives. This in turn dramatically relieves the production problems for the lens crystals.
In a quite different kind of embodiment of the invention, it was surprisingly found that in DUV microlithography with 248 nm or 193 nm, an aging process termed xe2x80x9ccompactionxe2x80x9d occurs in quartz glass during long-term operation; in it, the material becomes densified and consequently the refractive index and the shape of the lens become changed. This of course worsens the imaging power of the objective. Besides compensation by positionable elements, it was known that the most highly loaded image-side lenses concerned can be made of crystal, preferably of CaF2, SrF2 or BaF2, which are substantially more stable against UV radiation.
Several embodiment examples of such a use of calcium fluoride lenses are contained in German Patent Application DE 19855157.6 of the Applicant, of the same priority date, which Application is incorporated herein by reference as part of the disclosure of the present Application.
BaF2 as well as SrF2 in this reference have the advantage of differing substantially less than CaF2 from quartz glass in their optical properties (see Roblin, as cited hereinabove)xe2x80x94although this is a disadvantage in the context of achromatization. The design changes of a projection objective on replacing quartz lenses by BaF2 or SrF2 lenses near the image are therefore minimal. The projection objective is thus optimized by the use of two crystalline materials: CaF2 for achromatization, and BaF2 or SrF2 against compaction.
For a 157 nm objective, purely refractive and made from one material, accordingly CaF2, laser bandwidths of less than 0.1 pm are necessary, depending on the aperture and size of image field.
It cannot be expected that this value can easily be attained on changing from 193 nm to 157 nm. Once again everything is more demanding: permeability of material, layer availability, lattices for the laser components.
According to the invention, a material has been found in BaF2 which is transparent and isotropic at 157 nm, which has a markedly higher dispersion at 157 nm than does CaF2, and which can be supplemented with this for achromatization. BaF2 first completely absorbs at about 130 nm. The closeness of the absorption edge to 157 nm is responsible for the rapid course of the change in refractive index (strong dispersion) at 157 nm. This similarly holds for other fluorides such as SrF2.
Achromatization at 193 nm is established by the combination of CaF2 and quartz glass. BaF2 has an only slightly higher dispersion than CaF2 and is situated uselessly, so to speak, between the dispersion of CaF2 and quartz glass.
The situation changes for 157 nm, since quartz glass shows increased absorption. According to the general opinion heretofore, there is now no suitable partner for CaF2.
This is not the case. The dispersion distance between CaF2 and BaF2 at 157 nm is indeed smaller than that between CaF2 and quartz glass at 193 nm, but a still very good partial achromatization, to a similar level as at 193 nm, e.g., 50% color longitudinal error reduction, is always possible by a moderate use of BaF2.
At 193 nm only a partial achromatization is generally carried out, in order to keep the volume of CaF2 used small, for reasons of cost, availability, and material properties. At 157 nm, it is desired to keep the volume of the partner BaF2 small, since it has a higher specific weight, and BaF2 lenses therefore deform more strongly due to gravity.
At 193 nm as much as possible may be made of quartz glass; at 157 nm, as much as possible of CaF2. Since the number of positive lenses in refractive lithographic objectives is markedly larger than that of negative lenses, it would be advantageous at 193 nm if quartz glass had a small dispersion. However, the situation is reversed: CaF2 has the smaller dispersion and cannot, or should not, be used in all the positive lenses. Thus positive lenses are made of quartz glass, which depresses the degree of achromatization.
At 157 nm it is likewise desirable that the preferred material, here CaF2, has a smaller dispersion than the partner.
In contrast to 193 nm, this is the case with BaF2 at 157 nm. Nearly all the lenses, certainly all the positive lenses, can be of the crown, namely CaF2. A few negative lenses are made of BaF2, or alternatively of SrF2, since the same holds qualitatively for this as for BaF2.
The above statements of course also hold for lenses in a catadioptric objective, particularly also for refractive partial objectives used therein. The objective according to the invention can also be catadioptric. It is important that lenses, and not only optical auxiliary elements such as deflecting prisms or plane plates consist of crystal.
If lithography is desired with a shorter wavelength than 157 nm and at the same time with very high numerical aperture, a multitude of already known problems is encountered in aggravated form. Firstly, it must be clear that a suitable bandwidth and output power of the light source is only to be expected at excimer laser wavelengths. The spectra of the rare gases indeed emit from about 60 nm, only these are broadband and hence only accessible to purely mirror systems.
Purely mirror systems with a really extended field of between 10 and 26 mm have no aperture greater than NA=0.6 up to now.
Excimer lasers can be operated at the following wavelengths below 157 nm:
As regards the question of materials, CaF2 is a known candidate, with very good transmission for 134 and 147 nm. For 134 and 147 nm, catadioptric lenses exclusively with CaF2 as the lens material are conceivable, and hence represent nothing new. To obtain refractive objectives with more aperture, such as 0.80/0.85/0.90, the wavelength 147 nm is opened up with the above-mentioned materials system for 157 nm: positive lenses predominantly of CaF2, and a few negative lenses of BaF2. Since the absorption edge of BaF2 is situated at about 134.5 nm, 147 nm is still separated from it by about 13 nm. This means an increased absorption, but however makes possible a more relaxed color longitudinal error correction, since BaF2 at 147 nm now has a stronger dispersion than at 157 nm, and indeed with a higher increase than CaF2, since the absorption edge of CaF2 is situated at 125 nm.
In other words, with equally good bandwidth of the laser of 157 nm and 147 nm, in a purely refractive objective a material pair of CaF2 and BaF2 provides a better result as regards color correction. The absorption losses are of course higher.
At 134 nm also, no material pair for achromatization of purely refractive systems was heretofore given. This was found according to the invention in the material pair CaF2xe2x80x94SrF2. The distance to the absorption edge of SrF2 at 130 nm has however to be classified as very small. The increased absorption only permits the crystal to appropriately appear in very thin and small negative lenses. Therefore such a system can only be realistic for small image fields, possibly a half field system (stitching).
CaF2 is also completely excluded at 126 nm, since the distance to the absorption edge is only 1 nm.
There remain as known materials, MgF2 and LiF. MgF2 is strongly birefringent at 126 nm and therefore unsuitable. LiF at 126 nm is indeed transparent, but is unsuitable for smaller diameters, since the radiation loading rises and the material is impermissibly changed thereby (transmission and refractive index). Even a catadioptric system does not sufficiently come through without material in the region of the highest aperture (before the image plane). Thus for 126 nm, high aperture lithography objectives with a large field could no longer be constructed.
A further material can now be given according to the invention, especially for the highly loaded small diameter. The configuration at 126 nm consists of a catadioptric lithography objective, which mainly consists of LiF. In the lenses which are loaded with a high light intensity, it consists however of the isotropic amorphous form of BeF2. The crystalline form is birefringent, similarly to crystalline quartz. The glassy, solidified form, similarly to quartz glass, is isotropic when similarly produced. Since BeF2 is markedly more laser-resistant at 126 nm than LiF, it is the suitable material in a few lenses either in the beam waist or several beam waists and/or on the image end of the objective. The small number of BeF2 lenses is lastly to be sought because BeF2 has to be classified as a powerful respiratory poison and as a weak contact poison. For the infrared region, there are long established production lines which control the handling of poisonous optical components. It is nevertheless advantageous to restrict the number of BeF2 lenses to the indispensable ones. Thus a refractive or catadioptric lithography objective is concerned, of at least one crystalline and one vitreous fluoride. BeF2 has to be produced and worked in the absence of water, since it tends to take up H2O, and the H2O immediately blocks the 126 nm wavelength.
Especially low H2O content BeF2 production also makes the he laser wavelength at 109 nm accessible. Both components in highly pure form, LiF and BeF2 make a catadioptric objective possible at 109 nm.
Optical materials with high dispersion are conventionally termed flint (glass), and those with low dispersion are termed crown (glass). The following compactly described combinations of materials are proposed according to the above-mentioned for the different DUV through VUV wavelengths.
where SrF2 is used for the smaller diameters, since it is more radiation-resistant than NaF.
The above-mentioned systems can have reflections eliminated with thin layer systems of MgF2 and LaF3. For 193 nm, SiO2 and Al2O3 are additionally suitable as antireflection layers.
The possibility of eliminating reflections is an important prerequisite for the attainability of multi-membered refractive objectives, since otherwise about 10% of reflection occurs per lens surface.
Since no antireflection layers are known for 126 and 109 nm, this is another reason why catadioptric systems with fewer (e.g., 3-5) lenses are preferred.
In the case of achromatization with LiF and NaF, a possibility presents itself of dispensing with crystal-gas boundary surfaces, and thus also with antireflection layers or reflection losses.
The thermal conductivity and the thermal expansion of both materials are very similar:
Thus a xe2x80x9ccemented memberxe2x80x9d can be produced by wringing, and contains one each of a + and a xe2x88x92 lens, or two + lenses and one xe2x88x92 lens. Since the refractive indices of the two crystals are very low and elimination of reflections is therefore difficult, this formation of a cemented member is particularly helpful.
The objective could furthermore consist of CaF2 lenses, as well as individual cemented members of this kind.
Wrung members of CaF2 and BaF2 are also possible:
It is just the excellent thermal conductivities of the crystals in comparison with glasses that make such wringing appear more reliable, especially when there is different absorption (and thus heating).
As further crystals, the mixed fluoride crystals are above all suitable, among them those with alkali or alkaline earth and other elements, such as tin, zinc, or aluminum. High dispersion and good light resistance at high transmission in the VUV are the selection criteria here, with the avoidance of birefringence.
The above statements naturally also hold for lenses in a catadioptric objective, particularly also for refractive partial objectives used in these. The objective according to the invention can thus also be catadioptric. It is important that lenses, and not only optical auxiliary elements such as deflecting prisms or plane plates, consist of crystal.